1. Field of the Invention
This invention is directed to optical spectrum analyzer devices, in general, and to an optical spectrum analyzer optical signal processor which includes a photodetector array for processing the time of arrival information, as well as the frequency, of multiple signals, in particular.
2. Prior Art
In many instances of signal detection, it is known to receive and analyze signals generated by another source and then use this detection as a means for tracking, identifying, locating and/or otherwise gathering information about the source. This information gathering can be done in terms of overt or covert activities. The signals from the sources can be encrypted or "in the clear" (i.e. uncoded). However, when the operations are covert, the signals are usually encrypted.
Moreover, in this type of situation which is used for various kinds of security reasons, one of the techniques which is used to thwart the would be detectors is to use frequency hopping. This is a well known technique wherein the signals from the source (whether coded or not) are generated at a first frequency for a period of time, switched to a different frequency for another period of time, switched to a third frequency for another period of time and so forth. This frequency switching may be and in a secure situation, typically, is on a random or pseudo-random basis (although the intended receiver knows and uses the same frequency hopping patern).
Obviously, the unintended detecting party will lose some information when the source begins the frequency hopping process. That is, the interceptor of the information will lock onto the first frequency and do whatever detecting techniques and operations are appropriate. However, when the sending party switches frequencies, the intercepting party will lose contact until it finds the new frequency on which the sending party is operating.
With sophisticated equipment, this frequency hopping can be detected and the information loss can be minimized. Consequently, the sending party is required to use its own highly sophisticated equipment to offset the intercepting party's capability. Frequently, this means that the sending party must use equally sophisticated techniques and/or equipment. One of the simplest techniques is to switch frequencies very quickly so that the intercepting party will not have sufficient time to re-establish contact in its intercepting and tracking operations. Moreover, even if the intercepting occurs fairly rapidly, a significant amount of information is lost during each of these re-acquisition periods.
In the past, the frequency hopping detection schemes have been provided in both the electronic and optical apparatus embodiments. Each of these technologies has advantages to recommend it. However, the instant discussion is limited to optical systems.
Optical signal processing equipment is well known in the art. Many of these optical processors are capable of handling broadband optical information.
To date, the known optical processing devices or systems are capable of detecting information which is related to the frequency hopping techniques described above. However, the known systems currently available are quite limited in the operations which they can perform. The known systems are limited both in terms of frequency response and the ability to analyze information concerning the exact time of arrival of the received signal.
Most of the known systems use conventional electronic equipment such as fast scanning, superheterodyne receivers; compressive receivers; and crystal detectors. However, these systems are still limited in the operating characteristics thereof.
Fast scanning superheterodyne receivers have good sensitivity as well as good bandwidth and frequency resolution but low probability of intercept and poor ability to determine time of arrival for frequency hop and/or pulse type emissions. Compressive receivers have good sensitivity, and reasonable bandwidth, but only a moderate probability of intercept, low frequency resolution and poor ability to determine time of arrival information. Crystal detectors have excellent bandwidth and high probability of intercept and the ability to determine time of arrival. However, they suffer from poor sensitivity, total inability to determine frequency, and cannot discern the arrival of simultaneous signals of different frequencies.